In my limited experience with SW, yes, your in-laws will have to wait. We flew SW in September, and prior to beginning pre-boarding, they repeatedly announced the policy was that only the child and parent(s) were allowed to pre-board, any other members of their party, such as grandparents, had to get in the proper line for their boarding pass (A, B or C). Despite this being announced numerous times, there was a grandmother who remained in the pre-boarding line with her daughter and grandchild. When they got to the front of the line, grandma was told she could not pre-board with them, and was sent to the proper line for her boarding pass. She would have been better off had she been in the proper line to begin with, since now she was the back of the line for her boarding group.